The present invention relates to a foamed thermoplastic resin sheet manufacturing device which performs expansion under a reduced pressure.
As a conventional method for producing a foamed thermoplastic resin sheet (hereinafter sometimes referred to simply as xe2x80x9csheetxe2x80x9d), a manufacturing method wherein a thermoplastic resin and a foaming agent are melted and kneaded in an extruder and extruded through a die into under the atmospheric pressure has been well known. To obtain a highly foamed sheet by this manufacturing method, however, there are inconveniences that a great amount of a foaming agent is needed, and that cells become more coarse as the sheet expands, thereby deteriorating a strength of the sheet.
As a method which has such inconveniences solved, a method in which the foamy thermoplastic resin material extruded from an extruder is passed through a vacuum device so that the foamy thermoplastic resin is further expanded has been practiced.
For example, the Japanese Publication for Laid-Open Patent Application No. 54215/1990 (Tokukohei 2-54215) (the Japanese Patent No. 1639854) discloses an arrangement wherein a roll-like haul-off machine is installed in a vacuum chamber so that the sheet-like foamy thermoplastic resin material is extruded through a die to the vacuum chamber so as to expand and the material thus foamed is hauled by the haul-off machine.
The Japanese Examined Patent Publication 29328/1983 (The Japanese Patent No. 1199174) discloses an arrangement in which a sealing member for ensuring reduction of a pressure in a vacuum chamber is provided at an outlet of the vacuum chamber and a roll-like haul-off machine is installed behind the vacuum chamber so as to haul a sheet-like foamy thermoplastic resin material which is extruded through a die to the vacuum chamber thereby expanding.
Incidentally, in the present specification, a thermoplastic resin expanded under a reduced pressure at a first expansion stage is referred to as xe2x80x9cfoamy thermoplastic resin,xe2x80x9d and the foamed thermoplastic resin further expanded under the reduced pressure to a completely expanded state, obtained after or immediately before curing, is referred to as xe2x80x9cfoamed thermoplastic resin.xe2x80x9d A thermoplastic resin in a state of being expanded under a reduced pressure is to be classified as the former xe2x80x9cfoamy thermoplastic resin.xe2x80x9d
By the conventional arrangements described above, foamed thermoplastic resin sheets with only one thickness are produced. Therefore, to produce foamed thermoplastic resin sheets with various thicknesses, a discrete manufacturing device has to be prepared for each thickness.
The manufacturing device disclosed by Tokukohei 2-54215, however, has a drawback in that installment of the haul-off machine in the vacuum chamber causes the sealing mechanism in the vacuum chamber to become complicated, and makes the manufacturing device bulkier.
Further, regarding the manufacturing method disclosed by Tokukosho 58-29328, since the foamed sheet, while being hauled by the haul-off machine, pushes the sealing member at the outlet of the manufacturing device, cells in the foamed sheet are crushed and surfaces of the sheet are scarred. Note that the drawback in that cells of the foamed sheet are crushed is more remarkable in the case of Tokukohei 2-54215.
Therefore, though usually cells growing long in the thickness direction of the sheet are obtained in the case of expansion under a reduced pressure, such an effect of growth of cells due to pressure reduction is not sufficiently achieved in the foregoing prior art, and cells emerging in this case are cells growing long in the direction orthogonal to the sheet thickness direction, that is, in the sheet width direction or in the extrusion direction, like in the aforementioned case. Presence of a number of such cells in a sheet leads to a problem that the sheet cannot be made thicker.
The present invention was made in light of the foregoing problems, and the object of the present invention is to provide a manufacturing device of a foamed thermoplastic resin sheet which is capable of manufacturing foamed thermoplastic resin sheets with various thicknesses.
To achieve the foregoing object, a manufacturing device of a foamed thermoplastic resin sheet of the present invention is characterized by comprising (i) an extruder for melting and kneading a thermoplastic resin and a foaming agent to form a melted/kneaded mixture, and extruding the melted/kneaded mixture, (ii) a die provided at a front end of the extruder, for forming the melted/kneaded mixture into a sheet-like foamy thermoplastic resin material, (iii) a vacuum chamber in which the foamy thermoplastic resin material extruded through the die expands under a reduced pressure, and (iv) a facing-wall section composed of a pair of walls of the vacuum chamber which face each other in a thickness direction of the foamy thermoplastic resin material, at least one of the walls being a movable wall provided so as to move in directions in which the walls approach and separate each other.
With the foregoing arrangement, the melted/kneaded mixture of the thermoplastic resin and the foaming agent is extruded through the die by an extruding operation by the extruder, thereby becoming a sheet-like foamy thermoplastic resin material. The foamy thermoplastic resin material further expands under a reduced pressure in the vacuum chamber, thereby becoming the formed thermoplastic resin sheet.
Here, the thickness of the foamed thermoplastic resin sheet is determined depending on a dimension of the vacuum chamber in the foamed thermoplastic resin sheet thickness direction. In other words, the foamed thermoplastic resin sheet expands to a thickness equivalent to the dimension of the vacuum chamber in the foamed thermoplastic resin sheet thickness direction.
On the other hand, in the facing-wall section composed of a pair of walls of the vacuum chamber which face each other in the thickness direction of the foamy thermoplastic resin material, at least one of the walls is a movable wall provided so as to move in directions in which the walls approach and separate each other. Therefore, the thickness of the foamed thermoplastic resin sheet in the vacuum chamber can be changed by moving the movable wall, and as a result, the foamed thermoplastic resin sheet can be formed to various thicknesses.
Another object of the present invention is to provide a manufacturing device of a foamed thermoplastic resin sheet which is capable of producing a thick sheet whose surface condition is excellent and whose good foaming state under a reduced pressure is maintained thereby ensuring a high foaming ratio.
To achieve the foregoing object, a manufacturing device of a foamed thermoplastic resin sheet of the present invention, arranged as above, is further arranged as follows.
Namely, a sealing member is provided at least on the movable wall in the facing-wall section, at an outlet of the vacuum chamber, so as to seal a space between the wall and a foamed thermoplastic resin sheet which is obtained as a result of expansion of the foamy thermoplastic resin material under a reduced pressure and comes out of the vacuum chamber.
The foregoing arrangement ensures that a set desired pressure is kept in the vacuum chamber, without a great force applied from the sealing members to the foamy thermoplastic resin material and the foamed thermoplastic resin sheet, at all times since the start of pressure reduction.
For example, the gap between the walls in pair of the facing-wall section is narrowed in a period prior to pressure reduction in the vacuum chamber, i.e., when expansion of the sheet-like foamy thermoplastic resin material due to pressure reduction does not yet start and the material has a small thickness, whereas the gap is widened after the pressure reduction in the vacuum chamber starts thereby causing the sheet-like foamy thermoplastic resin material to expand due to the reduced pressure and have a greater thickness, that is, when the sheet-like foamy thermoplastic resin material becomes the foamed thermoplastic resin sheet. Therefore, before and after the start of the pressure reduction in the vacuum chamber, the respective relationships between the walls and the sheet-like foamy thermoplastic resin material or the foamed thermoplastic resin sheet, that is, the respective position relationship between the sealing members and the sheet-like foamy thermoplastic resin material or the foamed thermoplastic resin sheet in the thickness direction of the sheet-like material (or sheet) can be maintained substantially unchanged. This allows the sealing members to be made of a soft and flexible material, and the sealing members thus formed by no means scar the surfaces of the foamed thermoplastic resin sheet, nor crush cells in the foamed thermoplastic resin sheet.
In contrast, by an arrangement in which the facing-wall section does not have a movable wall, it is required to make the sealing members of a material with a relatively high rigidity so as to maintain a desirably reduced pressure in the vacuum chamber before and after a change of the thickness of the sheet due to expansion under the reduced pressure. In this case, an excessively great force is applied from the sealing members to the foamed thermoplastic resin sheet with a greater thickness due to expansion under the reduced pressure, whereby the surfaces of the foamed thermoplastic resin sheet tend to be scarred by the sealing members, or cells in the sheet tend to be crushed.
Consequently, with a simple arrangement of the present invention in which sealing members are provided as sealing means for maintaining the reduced pressure in the vacuum chamber, a sheet whose surface is not scarred by the sealing members and which maintains a good foaming state under a reduced pressure thereby having a high foaming ratio and being formed thick can be easily obtained.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.